1. Field
This application relates to methods for processing data obtained from nuclear magnetic resonance (NMR) equipment that is used to investigate earth samples containing hydrocarbons. This application more particularly relates to methods for processing NMR tool data obtained from investigating formations containing hydrocarbons that exhibit short relaxation times in order to obtain information regarding one or more characteristics of the formation, although it is not limited thereto.
2. State of the Art
Nuclear magnetic resonance (NMR) tool are used in oilfield applications to enable characterization of petrophysical properties. Low-field relaxation measurements made by the NMR tools are often dominated by short relaxation components, typically on the order of the echo-spacing of the NMR pulse sequence. For example, bulk relaxation of heavy oils often falls below 100 msec. Recent studies indicate that Barnett gas shales contain organic matter in the form of kerogen in various stages of maturation, see C. H. Songergold, et al., “Microstructural Studies of Gas-Shales”, SPWLA 50th Annual Logging Symposium, 2009, and that there is significant porosity in small pore sizes of the organic matter varying between 5-1000 nm. This results in significant T2 relaxation below 10 msec. Laboratory studies on Haynesville gas-shale cores also provide experimental evidence that fluid is restricted in nanopores resulting in T2 relaxations that are smaller than a few milliseconds. R. Kaushik et al., “Characterization of Gas Dynamics in Kerogen Nanopores by NMR”, SPE-147198, 2011.
Traditionally, pulse sequences and data inversion are optimized for magnetization data that fall in the middle of the T2 relaxation spectrum; i.e., between 50 ms and 500 ms. To enhance the precision at short relaxation times, data has been acquired according to an “enhanced precision mode” (EPM—a Trademark of Schlumberger). See, D. McKeon et al. “An improved NMR tool design for fasting logging”, SPWLA, 1999. This mode involves acquiring magnetization data at at least two different wait times often referred to as “main” and “burst”. In addition, improved precision in porosity determinations has been obtained by increasing the number of repeats, and thus improving the signal-to-noise ratio (SNR). See, P. Hook et al., “Improved Precision Magnetic Resonance Acquisition: Application to Shale Evaluation”, SPE—146883, 2011.